Similar in concept to java.awt.image.RGBImageFilter, but designed
to run more efficiently when filtering large images.

As with RGBImageFilter, you only have to implement a single routine
to use the filter. However, RGBImageFilter's routine filters a single
pixel at a time. This means a lot of routine-calling overhead.
RGBBlockFilter filters a block at a time.

Here's a sample RGBBlockFilter that makes an image translucent
by setting all the alpha values to 0x80: